Apparatus for image formation on the inside of a cylinder

ABSTRACT

A roller electrode is positioned in contact with the inside surface of a transparent cylindrical electrode forming a nip. Photoelectrophoretic ink in the vicinity of the nip is exposed to light through the transparent cylinder while a voltage gradient is applied between the electrodes. The image formed on the inside of the electrode is viewed and/or transferred to a web fed axially into the cylinder and deflected toward the image.

United States Patent Ztucker Feb. 15, 1972 [54] APPARATUS FOR IMAGE FORMATION ON THE INSIDE OF A CYLINDER [72] Inventor: Edwin Zucker, Rochester, N.Y.

[73] Assignee: Xerox Corporation, Rochester, NY.

[22] Filed: Dec. 29, 1969 [21] Appl. No.: 888,545

[52] US. Cl ..204/300, 204/181, 96/ 1.3 [51] Int Cl ..B0lk 5/02, C23b 13/00 [58] Field of Search ..204/299, 300, 181; 96/1 R,

[56] References Cited UNITED STATES PATENTS 2,829,050 4/1958 Huebner ..96/1.3

2,940,847 6/1960 Kaprelian ..96/1.2 3,427,242 2/1969 Mihajlov ..204/300 Primary Examiner-John H. Mack Assistant Examiner-A. C. Prescott Att0rneyJames J. Ralabate, David C, Petre and Michael H. Shanahan [57] ABSTRACT A roller electrode is positioned in contact with the inside surface of a transparent cylindrical electrode forming a nip. Photoelectrophoretic ink in the vicinity of the nip is exposed to light through the transparent cylinder while a voltage gradient is applied between the electrodes. The image formed on the inside of the electrode is viewed and/or transferred to a web fed axially into the cylinder and deflected toward the image.

17 Claims, 5 Drawing Figures PAIENTEUFEB 15 I972 SHEET 1 [IF 2 FIG. 5

INVENTOR. EDWIN ZUCKER ATTORNEY APPARATUS FOR IMAGE FORMATION ON THE INSIDE OF A CYLINDER BACKGROUND OF THE INVENTION This invention relates to imaging systems and in particular to improved photoelectrophoretic imaging methods and apparatus.

In the photoelectrophoretic imaging process, an image is formed from animaging suspension or ink by subjecting the ink to an electric field and exposing it to activating electromagnetic radiation, e.g., visible light. The imaging suspension is comprised of light sensitive particles suspended within an insulating liquid carrier and believed to bear a net electrical charge while in suspension. Normally, the ink is placed between injecting and blocking electrodes used to establish the electric field and is exposed to a light image through one of the electrodes which is at least partially transparent. According to one theory, particles attracted to the injecting electrode by the electric field exchange charge with the injecting electrode when exposed to light and migrate under the influence of the field through the liquid carrier to the blocking electrode. As a result of the migration, positive and negative images are formed on the two electrodes. The blocking electrode is covered'with a dielectric material to prevent charge exchange with the particles and thereby prevent the particles from oscillating back and forth between the two electrodes.

The photoelectrophoretic imaging process is either monochromatic or polychromatic depending upon whether the light sensitive particles within the liquid carrier are responsive to the same or different portions of the light spectrum. A

full color polychromatic system is obtained, for example, by

using cyan, magenta and yellow colored particles which are responsive to red, green and blue light respectively. An extensive and detailed description of the photoelectrophoretic process is found in U.S. Pat. Nos. 3,384,565 and 3,384,484 to Tulagin and Carreira, 3,383,993 to Yeh and 3,384,565 to Clark and the disclosures of these patents are expressly incorporated by reference into the present disclosure.

The above identified patents illustrate imaging systems in which one electrode is a roller or cylinder and the other a transparent flat plate. One advantage of these electrode configurations is that they permit the use of quite simple exposure mechanisms. In systems of the type disclosed in the Mihajlov U.S. Pat. No. 3,427,242, both electrodes are cylinders that contact at outside surfaces defining a nip in which image formation takes place. Images are formed in a line by line fashion on the outside of the cylinders from photoelectrophoretic ink in the vicinity of the nip subjected to field and exposed to light. The presently known mechanisms available for exposing the ink between two cylinders are complex as compared to those used with systems having a transparent fiat plate electrode. One reason for the additional complexity is that in systems such as illustrated in the Mihajlov patent, the light is first directed into the center of a transparent cylinder and then outward toward the other cylinder rather than directly through one electrode toward the other. On the other hand, systems such as illustrated in the Mihajlov patent are more convenient for the continuous, rapid serial production of a plurality of images as is desired for commercial applications of the photoelectrophoretic process.

Accordingly, it is an object of the present invention to devise photoelectrophoretic imaging methods and apparatus that incorporate the beneficial features of each of the foregoing noted types of imaging systems.

' Another object of the present invention is to devise novel methods and apparatus for displaying photoelectrophoretic images.

Another object in keeping with the foregoing object is to devise means for displaying a photoelectrophoretic image without exposing the image to the environment of the viewer.

Yet another object of this invention is to devise a novel compact photoelectrophoretic imaging system.

Still another object of this invention is to be able to employ simple exposure mechanisms with a photoelectrophoretic imaging system having two cylindrical electrodes.

Even a further object of this invention is to devise methods and apparatus for transferring an image on the inside of a rotating cylinder to a support member.

The foregoing and other objects of the present invention are accomplished by utilizing two cylindrical electrodes of different diameter (the smaller is hereafter called the roller) which are the system injecting and blocking electrodes. The roller electrode is positioned internal of the cylinder against its inside surface. The cylinder is transparent with image formation occurring in the vicinity of the nip between roller and cylinder upon applying a voltage gradient between the electrodes and directing electromagnetic radiation through the transparent cylinder to photoelectrophoretic ink in the nip. The image formed may be viewed directly by an observer from outside the cylinder. The image is on the inside surface of the cylinder and is totally protected from the environment outside the cylinder by the very electrode on which it is formed. Alternately and/or additionally, the image may be transferred to a permanent record sheet or transfer member. This is accomplished by feeding a web into the cylinder along its axis, by deflecting the web downward into the nip between a transfer roller and the cylinder and by once again deflecting the web parallel to the cylinder axis for its exit from the cylinder.

DESCRIPTION OF THE DRAWINGS Other objects and features of the present invention will be apparent upon a further reading of the present description and from the drawings which are:

FIG. 1 is an end elevation view of the photoelectrophoretic DESCRIPTION OF THE INVENTION The injecting and blocking electrodes of the present photoelectrophoretic imaging system are the cylinderor drum 1 and the roller 2. Either the cylinder or the roller can be the injecting electrode, however in the following description, the cylinder is an injecting electrode and the roller a blocking electrode. One reason for this particular selection of functional roles for the cylinder and roller is that it is preferred that the cylinder be transparent in order to simplify the exposure apparatus. The presently known exposure apparatus for projecting a light image inward through a transparent cylinder are less complicated and generally more efiicient than apparatus for projecting an image outward through the cylinder. Once it is established that the cylinder is to be transparent rather than the roller the selection of functional roles is quite easy. Generally, it is preferred that the transparent electrode be a highly efficient charge exchange device, i.e., an injecting electrode. Since positive images (positive" being used in the classical photographic sense) are normally produced on transparent electrodes in a subtractive image forming process, the present system is particularly advantageous for use with polychromatic inks. Mono-chromatic inks may also be used in the present system and in such cases the functional roles of the cylinder and roller may be interchanged to accommodate the monochromatic ink. Similarly, it may be preferable for the roller to be transparent rather than the cylinder. In any event, such modifications are clearly within the scope of the present invention wherein images are formed on the inside or concave surface of a cylindrical electrode.

In FIG. 1, the cylinder 1 includes the metal support cylinder or drum 3 having a segment cut away to accommodate the transparent cylindrically shaped glass 4. The inside or concave surface of the glass is coated with an electrically conductive layer of tin oxide 5 that is transparent to activating electromagnetic radiation. The glass is a structural support for the transparent conductive layer. An alternate structure for cylinder 1 is a cylinder constructed solely of glass having the tin oxide over substantially its entire inside surface. The cylinder is journaled for rotation about its geometric axis 7 j and is coupled to an appropriate drive mechanism for effecting its rotation.

The roller 2 is constructed of an electrically conductive inner core 8 and an electrically insulating outer layer 9. The roller, also termed the imaging roller or electrode, is journaled for rotation about its geometric axis 10 and is coupled to an appropriate drive mechanism for effecting its rotation. The periphery of the imaging roller contacts the inside surface of the cylinder forming a nip. The nip is the area defining the interface between the imaging roller and cylinder whether they are contacting or separated by a layer of ink. The cylinder axis 7 and the roller axis 10 are substantially parallel, and during the image forming process, remain fixed relative to one another so that the nip extends substantially the width of the roller and drum. In addition, the two electrodes are oriented to allow gravity help build a reservoir from a fluid, such as the ink, at the entrance to the nip. The reservoir of fluid tends to suppress corona current between the electrodes which has been found to adversely affect certain photoelectrophoretic inks. There is less concern with the corona current at the exit to the nip since the corona current generated in that area is believed beneficial to the images. Consequently, the mating of convex (the roller) and concave (the cylinder) surfaces is advantageous for photoelectrophoretic systems because the generation of potentially adverse corona at the nip entrance is suppressed by a fluid reservoir while the generation of potentially beneficial corona at the nip exit is encouraged because the distance between the electrodes increases slowly at points away from the nip.

The roller and cylinder are rotated at angular velocities selected to give points on their peripheries in the vicinity of the nip substantially 0 relative velocities. Consequently, when a voltage gradient is applied between the electrodes and a light image is directed through the transparent portion of the cylinder into the nip, an image is formed on the inside surface of cylinder 1 and on the roller from ink in the nip exposed to the light and subjected to the field. The voltage gradient applied between electrodes establishes the electric field effecting migration of ink particles struck by light. The gradient is applied by commutating the tin oxide layer 5 on the cylinder to an electrical ground 6, for example, and by commutating the imaging roller to a high voltage potential source (generally in the order of:3-4,000 volts) such as voltage source 12.

The light image to which the ink is exposed is activating electromagnetic radiation in imagewise configuration. Several well known and relatively simple exposure mechanisms may be used. By way of example, FIG. 1 illustrates a transparency slit scan exposure mechanism 13 including a lamp 14, light stop 15 and lens 16. The light generated by the lamp passes through the transparency l8 and the slit or linear aperture in the light stop and is projected to the nip by the lens. The transparency is moved past the aperture synchronously with the rotation of the cylinder to construct in a line by line fashion a complete photoelectrophoretic image of the original on the tin oxide surface 5. Images of opaque originals are projected to the nip in a similar fashion. The opaque original is moved synchronously with the cylinder past lamps that flood a linear element of the original and lens and/or mirrors that project the light from the flooded areas to the nip. Other slit scan systems employing translating or rotating mirrors for projecting linear elemental images of either transparent or opaque originals are known and may be used in the present system. One advantage of this invention is that the location of the imaging roller electrode inside a transparent cylindrical electrode permits the use of a variety of conventional well-known exposure mechanisms.

The photoelectrophoretic ink is applied to the nip by means of the inking roller l9joumaled for rotation in the walls of ink tank 20. The inking roller is coupled to an appropriate drive mechanism for effecting its rotation and the rotation of the intermediate roller 21. Roller 21 is at least partially submerged in ink 22 stored in the tank and coats the surface of roller 19. The inking roller in turn coats the tin oxide surface 5 of the cylinder. The rotation of the cylinder carries the ink to the nip between electrodes where a reservoir of ink is formed. Alternately, the inking roller 19 and tank 20 could be positioned adjacent roller 2 to coat the surface of the imaging roller which in turn would carry the ink into the nip.

In the present embodiment, the image formed on the inside of the cylinder is used rather than the image formed on the roller. Accordingly the cleaning apparatus including knife edge 41, roller 42, tank 43 and insulating liquid 50 is used to remove the image formed on the imaging roller electrode 2. The knife 41 is supported by the tank 43 and contacts the periphery of roller 2 to scrape ink off the roller. The cleaning roller 42 is journaled for rotation in tank 43 and is positioned to coat the surface of roller 2 with the insulating fluid 50. The fluid 50 is normally the same fluid as the carrier liquid used in the ink 22. The fluid 50 is carried to the nip entrance where it alone or in combination with ink 22 forms a reservoir for corona current suppression and provides a medium for ink particle migration.

The image formed on the inside of the cylinder electrode 1 can be viewed directly by an observer. In addition or alternately, the image can be transferred to a permanent record member. The immediate problem with transferring the image is the task of bringing a transfer member inside the cylinder, placing it against the image and advancing the transfer member out of the cylinder. This problem is solved in the present system by feeding a web 25 into the cylinder parallel to its axis 7. The web is deflected from its generally axial direction of travel by the turning bar 26. The web passes between the tin oxide layer on the cylinder and the transfer roller electrode 27 to effect the transfer of an image on the cylinder to the web. The web exits the cylinder axially when deflected 90 from its direction of travel after leaving the transfer roller by turning bar 28.

The turning bars 26 and 28 are shown in side, front and plan views in FIGS. 1, 2, and 3. The turning bars are mounted inside the cylinder 1 at substantially a 45 angle to the axis 29 of the transfer roller electrode. The web 25 is folded over the input turning bar 26 to deflect the web downward toward the transfer roller. The web is folded'over the exit turning bar 28 to deflect the web out of the cylinder. The 45 angle is selected because the web 25 enters and exits the cylinder generally parallel to the cylinder axis 7. If the web 25 is fed into the cylinder at an angle relative to axis 7, the turning bar 26 is moved from the 45 mounting angle to an angle that enables it to deflect the web to a direction of travel generally perpendicular to the axis of the transfer roller. Similarly, if the web is to exit the cylinder at an angle to the cylinder axis, the turning bar 28 is moved from the 45 mounting angle to an angle selected to give the desired direction of travel for the web as it makes its exit. The exit turning bar 28 may be oriented 90, for example, to the input turning bar 26 in order to exit the web through the end of the cylinder opposite the end through which it entered.

The turning bars are cylindrically shaped rods or shafts of relatively small diameter compared to the imaging or transfer roller electrodes. The turning bars do not rotate but can be mounted for rotation without altering their function of changing the direction of web travel. However, it is preferred in the present embodiment to lock them against rotation. One reason bar 26 is locked against rotation is that it also serves as a conduit for a fluid that wets web 25 prior to the transfer of an image to it. Turning bar 26 has a cavity 31 extending its length which is coupled to the hose or pipe 32. Pipe 32 is in fluid communication with an appropriate hydraulic system for pumping a fluid into the cavity 31. The fluid is electrically insulating and tends to suppress corona current between the transfer roller (or web) and cylinder. The fluid also increases the volume of carrier liquid available at the transfer roller nip and in that regard is helpful for ink particle migration. Normally, the fluid pumped into cavity 31 is the same fluid comprising the insulating carrier for the photoelectrophoretic ink. The fluid is applied directly to the web through a plurality of small holes 33 (FIGS. 4 and 5) drilled into the bar 26 along its length.

The electrically insulating fluid is applied to surface B of web (FIG. 5) which is the surface that receives the image. Surface B faces inward toward the holes 31 as the web passes over the turning bar 26, outward toward cylinder 1 as the web passes around the transfer roller 27 and again faces outward as the web passes over the turning bar 28 on its way out of the cylinder. Folding the web around turning bar 28 so that surface B faces outward prevents the image from being smeared or otherwise adversely effected as the web passes around bar 28.

The transfer roller electrode 27 is structurally similar to the imaging roller 2 in that it includes an electrically conductive inner core and an outer covering of electrically insulating material 36. The voltage potential source 37 is coupled to the core of the transfer roller. A voltage gradient between the transfer roller and cylinder 1 establishes an electric field capable of effecting removal of the charged ink particles composing an image from the cylinder toward the transfer roller. For the present system wherein the tin oxide layer 5 is coupled to electrical ground, the polarity of the voltage potential applied to the transfer roller is opposite that of the voltage potential applied to the imaging roller and is of generally the same magnitude (iii-4000 volts).

The web 25 is commonly a cloth or wood fiber paper having sufficient mechanical strength to resist tearing when fed into and out of the cylindersover the turning bars 26 and 28. The web could be constructed of an electrically insulating material carried on a conductive substrate and as such be the principal dielectric material for establishing the electric field between the transfer roller and cylinder. In such a case, the transfer roller need not have an insulating outer coating. The feeding of the web into and out of the cylinder is accomplished by suitable drive means (not shown) capable of advancing the web while maintaining sufficient tension in the web for folding it over the turning bars. An example of an appropriate drive mechanism is one wherein the web is continuously pulled from a spool by pinch rollers.

The image transferred to the web is fixed to the web prior to passing the pinch rollers or other drive mechanism. The portion of the web carrying the image is severed by a guillotine or knife positioned outside the cylinder to cut the web after it passes the pinch rollers or other web drive apparatus.

After the tin oxide layer 5 on the cylinder rotates past the transfer roller 27, it encounters the cleaning roller 38. Roller 38 has a fibrous material on its outer surface that removes residue ink from the cylinder primarily by virtue of its contact with the cylinder. The cleaning roller is rotated in a direction opposite to that of the cylinder to create a scrubbing action. A cleaning fluid 39 is stored in tank 40 to wet the surface of the cleaning roller which in turn helps to remove residue ink and other unwanted materials from the tin oxide layer 5. The cleaning fluid is commonly the same as that comprising the insulating carrier liquid for the ink. A voltage potential source may be coupled to the cleaning roller to establish an electric field between it and the grounded cylinder to help remove charged ink particles. The cleaning and transfer rollers comprise means for erasing an image formed on the cylinder.

The imaging roller 2, inking roller 19, the transfer roller 27 and cleaning roller 38 are all coupled to appropriate means (not shown) for moving them out of contact with the inside surface of the cylinder, specifically the conductive tin oxide layer 5. This feature is included so that an image formed in the nip between the imaging roller and cylinder can be exposed to a light image and subjected to a field a second or more times. With the transfer, inking and cleaning rollers out of contact with the cylinder, the image formed on the cylinder is undisturbed as the cylinder makes additional revolutions. In the meantime, the knife edge 41 and cleaning roller 42 in contact with the roller electrode 2 remove the ink particlestransferred to the image roller during the image forming process. The surface of the imaging roller is therefore free of ink particles and coated with insulating carrier liquid 50 when the image on the cylinder once again returns to the imaging roller. (Fluid 50 forms a corona suppressing reservoir at the nip entrance at this time.) The same light image is directed onto the image and field is applied causing particles not originally driven from the cylinder to the imaging roller to migrate at this time. The result is an image of improved quality. The transfer roller is now moved into contact with the cylinder to effect the removal of the twice exposed image. The cleaning roller 38 is moved into contact with the cylinder to remove residue ink particles and the inking roller is moved into contact with the inking roller to apply a fresh coat of ink to the cylinder for formation of the next image,

The location of the imaging, transfer, cleaning, and inking rollers internal the cylinder 1 makes for a highly compact system. The ends of the cylinder 44 and 45 (FIG. 3) are sealed by walls having appropriate cut outs for the web 25 to enter and exit and for the axles 46 of the various rollers. Sealing the cylinder traps vapors inside the cylinder thereby protecting the operator from possibly offensive odors and preventing the collection of vapors within the machine which may be a fire hazard, or a condensation hazard to exposed optical elements. Roller 38, in its normal cleaning function will remove condensed material from the tin oxide surface 5. The vapors are pumped out of the sealed cylinder by an appropriate fan or blower in fluid communication with the vapors or gases in the drum. In addition, the gas pressure inside the drum can be reduced below atmospheric pressure by the fan or blower. The reduced pressure inside the drum prevents leakage of the volatile vapors through the drum vapor seals.

Limitation by the specific embodiments of the invention as set forth in this application is not intended. Rather, it is intended that the claims apply broadly within the spirit and scope of this invention.

What is claimed is:

1. Photoelectrophoretic imaging apparatus comprising a transparent concave electrode supported for relative movement with a second electrode positioned inside the cylinder defined by the concave electrode, inking means for applying photoelectrophoretic ink between the electrodes, and I exposure means positioned outside the cylinder defined by the concave electrode for exposing ink between the electrodes to activating electromagnetic radiation whereby images are formed from ink exposed to radiation and subjected to electric field established by voltage potentials coupled to the electrodes.

2. The apparatus of claim 1 further including transfer means for transferring an image formed on the inside of the concave electrode to a transfer member.

3. The apparatus of claim 1 further including a transfer roller electrode supported for relative movement with the concave electrode for transferring an image on the concave electrode to a transfer member between said transfer and concave electrodes.

4. The apparatus of claim 3 further including folding means for deflecting said transfer member between generally an axial direction of travel to a radial direction of travel enabling the transfer member to be fed into the cylinder defined by the concave electrode, between said transfer and concave electrodes and back out of said cylinder.

5. The apparatus of claim 3 wherein said transfer member is a web andfurther including at least a first turning bar supported inside the cylinder defined by the concave electrode at an angle to the transfer roller to deflect said web from a generally axial direction of travel toward the transfer roller for its passage between the transfer and concave electrodes.

6. The apparatus of claim further including a second turning bar supported adjacent the first bar for deflecting a web coming from the transfer roller to a generally axial direction of travel for its exit from the cylinder defined by the concave electrode.

7. The apparatus of claim 5 wherein said turning bar is supported against rotation.

8. The apparatus of claim 5 wherein said turning bar includes means for directing a wetting solution onto the image receiving surface of the web.

9. The apparatus of claim 1 wherein said concave electrode includes a drum supported for rotation that is transparent to activating electromagnetic radiation over at least a portion of its periphery.

10. The apparatus of claim 9 wherein said second electrode includes a roller electrode journaled for rotation inside said drum adjacent the inside surface. forming a nip with the drum.

11. The apparatus of claim 10 further including a transfer roller electrode journaled for rotation inside said drum adjacent the inside surface for removing images formed on the drum.

12. The apparatus ofclaim 10 wherein said inking means includes means for coating one of said electrodes with photoelectrophoretic ink.

13. The apparatus of claim 12 wherein said electrodes are oriented to allow gravity to build a reservoir of ink at the entrance to the nip for suppressing corona current between said electrodes 14. The apparatus of claim 12 further including cleaning means positioned inside said drum adjacent the inside surface for cleaning said drum after an image is transferred from it 15. The apparatus of claim 12 further including cleaning means positioned inside said drum adjacent said roller electrode for cleaning the surface of the roller electrode.

16. The apparatus of claim 9 further including lid means to cover the ends of said drum for maintaining vapors inside the drum.

17. The apparatus of claim 16 further including means to reduce the pressure inside the drum below atmospheric pressure. 

1. Photoelectrophoretic imaging apparatus comprising a transparent concave electrode supported for relative movement with a second electrode positioned inside the cylinder defined by the concave electrode, inking means for applying photoelectrophoretic ink between the electrodes, and exposure means positioned outside the cylinder defined by the concave electrode for exposing ink between the electrodes to activating electromagnetic radiation whereby images are formed from ink exposed to radiation and subjected to electric field established by voltage potentials coupled to the electrodes.
 2. The apparatus of claim 1 further including transfer means for transferring an image formed on the inside of the concave electrode to a transfer member.
 3. The apparatus of claim 1 further including a transfer roller electrode supported for relative movement with the concave electrode for transferring an image on the concave electrode to a transfer member between said transfer and concave electrodes.
 4. The apparatus of claim 3 further including folding means for deflecting said transfer member between generally an axial direction of travel to a radial direction of travel enabling the transfer member to be fed into the cylinder defined by the concave electrode, between said transfer and concave electrodes and back out of said cylinder.
 5. The apparatus of claim 3 wherein said transfer member is a web and further including at least a first turning bar supported inside the cylinder defined by the concave electrode at an angle to the transfer roller to deflect said web from a generally axial direction of travel toward the transfer roller for its passage between the transfer and concave electrodes.
 6. The apparatus of claim 5 further including a second turning bar supported adjacent the first bar for deflecting a web coming from the transfer roller to a generally axial direction of travel for its exit from the cylinder defined by the concave electrode.
 7. The apparatus of claim 5 wherein said turning bar is supported against rotation.
 8. The apparatus of claim 5 wherein said turning bar includes means for directing a wetting solution onto the image receiving surface of the web.
 9. The apparatus of claim 1 wherein said concave electrode includes a drum supported for rotation that is transparent to activating electromagnetic radiation over at least a portion of its periphery.
 10. The apparatus of claim 9 wherein said second electrode includes a roller electrode journaled for rotation inside said drum adjacent the inside surface forming a nip with the drum.
 11. The apparatus of claim 10 further including a transfer roller electrode journaled for rotation inside said drum adjacent the inside surface for removing images formed on the drum.
 12. The apparatus of claim 10 wherein said inking means includes means for coating one of said electrodes with photoelectrophoretic ink.
 13. The apparatus of claim 12 wherein said electrodes are oriented to Allow gravity to build a reservoir of ink at the entrance to the nip for suppressing corona current between said electrodes.
 14. The apparatus of claim 12 further including cleaning means positioned inside said drum adjacent the inside surface for cleaning said drum after an image is transferred from it.
 15. The apparatus of claim 12 further including cleaning means positioned inside said drum adjacent said roller electrode for cleaning the surface of the roller electrode.
 16. The apparatus of claim 9 further including lid means to cover the ends of said drum for maintaining vapors inside the drum.
 17. The apparatus of claim 16 further including means to reduce the pressure inside the drum below atmospheric pressure. 